Ford Repair: ignition wires, oxygen sensors, fuel mileage

Question
QUESTION: I didn't include a model because my question is only general as to whether there are some new ignition wires that lack continuity when checked with an ohmmeter since checking 3 of the 6 on my 97 taurus are open(didn't check other 3 yet) but the car continues to run well except for a  drop in fuel mileage of 23 to 20 mpg.

ANSWER: What was the continuity. There should have been some. Even my 9mm Ford racing wires have some resistance. I understand your concern, 3 mph. over time adds up. A dirty PCV will cause a minimal loss of fuel efficiency as well as the air filter. I am assuming that you replaced the plugs as well. Did you use Motorcraft, or another after market plug?

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QUESTION: used ngk plugs, pcv and air filter are clean, replaced one upstream oxygen sensor that tested open presumably on the heater circuit with no result so switched it for the other one with continuity and no difference there either.  Wonder if the new one is good--any way to check it other than continuity?  Do you know how this thing actually measures oxygen and generates a current to the computer?  I understand how a thermocouple works by exposure to heat, but wonder how it generates current by oxygen content or lack of it.  Thanks much.

Answer
First things first. NGK spark plugs are for Nissans. There is no budging on this. Always, always use Motorcraft plugs if you want the same power and fuel economy. Try not to think about what plugs are cheaper, or which ones are on sale. Think about the long haul. As far as the oxygen goes, I don't know where you are going with this? Oxygen is measured before and after the catalytic converter by oxygen sensors. To give you a little extra knowledge, the oxygen sensors before the catalytic converters are heaters. They heat the exhaust approximately 100 degrees Fahrenheit. This enables the catalytic converters to do what they were designed to do.A drop in fuel mileage can sometimes be caused by a defective O2 sensor. If and when it does go bad, it will trip the check engine light. You didn't mention the check engine light was on, so it is probably not the O2 sensor. If the light is on, go to your local parts store and have the codes read for free.(PS)Don't forget to adjust the gap to the correct setting on the Motorcraft plugs. They are not preset, and do make a difference in mileage.